A certain kind of material is known to have good charge transport property and its application encompasses the following photorefractive effect. The photorefractive effect is one of the nonlinear optical effects and is a phenomenon in which a refractive-index of a substance is changed when light is absorbed by the substance. The following description explains a mechanism of the photorefractive effect. When two laser beams are interfered with each other in a medium having photoconductivity and second-order optical nonlinearity, the interference fringes are formed. In a bright region of the interference fringes, charge carriers are generated by a photo-excitation. Positive charge carriers are moved in the medium with an assistance of an externally applied electric field, and trapped in a dark region. As a result, the periodic distribution of charge density occurs in which the bright region is negatively charged and the dark region is positively charged, and thus a space field between them is formed. The space field induces the Pockels effect through a first-order electro-optical effect, and a periodic refractive-index grating is formed. A phase difference of φ is spatially caused between the refractive-index grating and the optical interference fringes, and therefore asymmetric energy transfer is observed between two light waves. Thus, optical amplification (optical gain) is obtained.
Such a photorefractive effect is expected to be applied to phase conjugation, imaging from a distorted medium, real-time holography, superimposed holographic recording, 3D display, a 3D printer, optical amplification, nonlinear optical information processing including an optical neutral network, pattern recognition, optical limiting, high-density optical data storage, and the like.
Conventionally, an inorganic crystalline material such as lithium niobate (LiNbO3) has been employed as a photorefractive material. However, the inorganic crystalline material has a problem of low processability. In recent years, organic photorefractive material has been developed.
The organic photorefractive material has many advantages as compared with the inorganic photorefractive material. The advantages encompass the easiness in optimization of a composition ratio and easy processability. Other advantages are large optical nonlinearity, low dielectric constant, low cost, light weight, pliability, and the like. Moreover, other important characteristics desired are that, depending on the purpose of use, the organic photorefractive material possesses long shelf life of data storage, and its optical quality and good thermal stability. Such organic photorefractive materials are becoming important for an advanced information communication technology. Among those, a carbazole type (see, for example, Patent Literature 1) and a triphenylamine type are known.
As a device for sequentially displaying a hologram by sequentially writing holographic images, various kinds of holographic display devices are known, and a photorefractive element including the photorefractive material as described above is used for the display devices. For example, Patent Literature 2 discloses a photorefractive element in which a layer containing a photorefractive material and a layer containing an electron-ion mixed conductor are provided between two transparent electrode substrates.